1. Technical Field
The present invention relates to an ink set. More specifically, the invention relates to an ink set suited for use in an inkjet recording system.
2. Related Art
Inkjet recording methods are printing methods in which printing is performed by making small ink droplets jet and adhere to a recording medium such as paper. Owing to the recent drastic developments in inkjet recording techniques, the inkjet recording methods are now used also for highly precise image recording (printing), which has until now been performed with photography or offset printing. Therefore, high quality recording is required not only for generally used plain paper and dedicated inkjet recoding paper (matte and gloss), but also for recording media such as printing paper.
An ink composition containing water, color material as colorant, water-soluble organic solvent, and surfactant, etc., is generally known as the ink used in the inkjet recording. Dye or pigment is used as the colorant. Particularly in a color ink, water-soluble dyes are often used because of their high color saturation, clarity, and water solubility, etc. However, lighffastness or gas fastness is generally insufficient in those dyes. The waterfastness of those dyes is also insufficient because of their water solubility, and therefore, matter recorded with an ink using water soluble dye has poor recorded image storage stability.
Meanwhile, water insoluble colorant has better waterfastness. Among water insoluble colorants, pigment is the color material having particularly excellent lighffastness, gas fastness, and waterfastness. Therefore, pigment inks utilizing those color material properties have been developed. For example, a water-based pigment ink in which pigment is dispersed with surfactant or polymer dispersant, one using self-dispersing pigment obtained by adding a water dispersible functional group to the pigment surface, and one using coloring particles obtained by covering pigment with water dispersible resin, have been proposed.
Some of those pigment inks have pigment particles (including pigment-containing coloring particulates) that are stably dispersed in an ink solution. However, during the process of the ink being dried after being attached to a recording medium such as paper, a structure in the ink changes with evaporation of water or volatile solvent. Therefore, a stable dispersion structure changes and the pigment is likely to aggregate. If that pigment is deposited on the recording medium, such as paper, that results in a problem of being unable to obtain high-quality images, because of the unsmoothed surface, reduced glossiness and less uniform printed images. Furthermore, there is another problem in that light reflected from the deposited pigment makes the printed images give off a hue different from the inherent one (bronzing), and images with a desired and uniform hue cannot be obtained.
The above-described (undesirable) hue change as a result of deposited color material sometimes occurs when using dye too, if it is used in high concentration. In order to prevent this, a method has been proposed where a water soluble polymer having a branched chain is applied to an ink to prevent dye molecules from crystallizing or aggregating during the drying of the ink, thereby reducing hue change (see JP-A-8-337748).
Also, various proposals have been made with respect to the quality of images made using a pigment ink. For example, limiting the particle diameter of a pigment dispersed in an ink (see JP-A-2002-356637), and containing a pigment together with a pigment derivative (see JP-A-2002-294134) have been proposed in order to improve glossiness, dispersiblity, etc.; containing polymer particulates (see JP-A-2001-123098) and containing inorganic particulates (see JP-A-11-209671) have been proposed in order to decrease blur and improve coloring ability; and an ink containing a pigment dispersion containing a water insoluble vinyl polymer has been proposed in order to improve printing density when applied to plain paper, and also improve glossiness when applied to inkjet-only paper (see JP-A-2005-041994).
However, the method disclosed in JP-A-8-337748, although it is effective in preventing the depositing of a color material included in a dye ink, cannot bring about sufficient advantages for a pigment ink. Furthermore, the method has had problems in terms of reliability, for example, a water soluble polymer is contained, and that decreases dispersion stability or increases ink viscosity.
In addition, there have also been unsolved problems when combining several known types of pigment ink to make an ink set and forming an image using that ink set. For example, even though each ink can exhibit good printing quality if it is used alone, if different color inks are mixed, pigments can aggregate in that color-mixed portion, reducing glossiness and making the printed images less uniform. The problem of gloss change also remains unsolved. Gloss change means a phenomenon where, if one touches a recorded material with one's finger or something, although the pigment is not removed, (finger) marks are left on the material; the colors at the touched portion seem different; or the gloss at the touched portion changes.